Greenhouse heating

[Digmeplot]

Does anyone have solar powered heating in there shed or greenhouse.


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[Capsid]

Yes, my greenhouse heats up when it's sunny.

Seriously, though how would that work? There's probably not enough sun in winter to heat it any more than what it will normally warm to.

[Digmeplot]

I was thinking that a trickle charge would go into a battery then at night a sensor would (like the sensor used in solar lights) turn in the power to turn on a heater to keep the temperature two to three degrees above the temperature outside?


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[Capsid]

I was thinking that a trickle charge would go into a battery then at night a sensor would (like the sensor used in solar lights) turn in the power to turn on a heater to keep the temperature two to three degrees above the temperature outside?


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I doubt you would be able to charge a battery sufficient to run a heater overnight to maintain that temperature. A heating engineer could probably work out the energy required. I think you need heat sink that you can heat up during the day.

[bazzaboy]

DigMe and C, You may find an earlier conversation on this topic of interest, see

http://www.growfruitandveg.co.uk/gra...use_30067.html

Although posted way back in 2009 the YouTube references still work and are interesting.

I stil use some approximation of this sun-warming heat-sink idea in a cold greenhouse, "solar" but no batteries. It does help even on cloudy days without direct sun as long as daytime temperatures get above freezing. Admittedly it's a bit hit and miss depending what the weather throws at us but hopefully better than nothing.

Interesting topic particularly as night-time temperatures are already getting uncomfortably close to frost....

[Digmeplot]

Thanks Bazzaboy I have really found this very useful.


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[bazzaboy]

Just to update this interesting thread.... You may have seen this morning (Sunday) in answer to a question "Watcha doin'?" zazen999 mentioned building a "rocket mass heater" and in looking at various designs I came across this little gem on YouTube (not really a "rocket mass heater" but related....) :
heating your home office for 8 pence a day- Potheadforkandle (TM) - www.keepturningleft.co.uk - YouTube
I wouldn't use this technique at night indoors because too much of a fire risk but in a greenhouse it must be as safe as any paraffin burner - and so simple. I don't see why you need 4 night-lights, 2 or perhaps even 1 in the right place might suffice.... Strangely, even though the device uses flower pots, no-one seems to have suggested using overnight in a greenhouse but I'm inclined to give it a try once real frosts start.

Of course the candle fumes/smoke will end up in the greenhouse (with a designed rocket mass heater they certainly wouldn't) but I can't see that they're going to harm the vegetation.

Looking at various designs of rocket mass heaters - which basically expel fumes, smoke and heat *sideways* rather than upwards to allow maximum transmission of heat - I was struck by the similarity with the Victorian greenhouse heating systems (coal/coke fires with gases/heat channelled down tunneled walls) that allowed them to grow pineapples!

There's also some correspondence about a gadget called a "Kandle Heeter" (worth a Google!) and also a "Blue Box" which is basically a little fan and a tube that takes warmest air from near the ceiling and releases it again at ground level (or in a greenhouse could be under a heat sink, as already discussed). A computer fan running on a battery at the end of length of drainpipe would probably suffice.....

Gosh, so many ideas, suddenly I wish it was colder...... Where are those frosts when you need 'em?

[Digmeplot]

I am going to give this idea a go, but am still researching the solar heating idea.


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[Bill HH]

Keep a cow in there, one cow gives off 3kw of heat an hour. Plus you get fertiliser ad- nauseam.

Actualy its not as daft as it sounds, a large poly tunnel could accomodate a few goats, and you get milk etc. Apparently in Switzerland farms used to keep the cows on the ground level of the house and the heat warmed their little toesies.

[veggiechicken]

Not just in Switzerland The Welsh Longhouses (and in Dartmoor and probably in the rest of the UK too) kept their cattle indoors through the winter, at one end of the longhouse - saved on the central heating bills

[polc1410]

Was gonna try and work through the science of heating the air but it gets very complex trying to factor in the surface area of glass etc. But these guys have already done some of the work for you:

Calculate the power required to heat a Greenhouse by Hartley Botanic

For a 4 x 6 greenhouse I got roughly 282W heater being required to lift the temperature from 0C to 3C. That doesn't sound much in domestic terms.

However, if you are storing power from a solar panel you'd be thinking of a 12V battery. P=IV so 282=I x 12 so I = 23.5Amps. Thats a good amount of current! If you only needed to apply that heat for 12hours a day (you won't!) you'd need 282Ah of battery power. You should only run a 12V battery to about 40% of its stated capacity or you'll kill it very quickly. So you'd need 6 x 120Ah Leuisure Batteries ( c �100 each).

Assuming you had the other 12hours as perfect sun (you won't and you wont have the capacity to store it from Summer when you might have done)... you'd need a 300W solar panel. I found a 100W on amazon for �100 but looks like you normally pay more than that. But lets assume you can do it for�300 you are now at �1000 and still wont have a system that can actually do what you need it to because (a) It wouldn't be charged enough after 1 night of use before the next because you wont get 12hr of perfect sun (b) You may need more than 12hours of heat (c) You may need more than 3C increase in temp. If you get a realtively cold (rather than stupidly cold) winter say -5C for 8 hours and you maybe need to get the air temp to 2C to avoid frosting and then the outisde temp rises to -2C for the rest of the day (16hours). You probably need close to 700W during the really cold bit and 400W in the 'warmer' bit. That takes you to needing 1000Ah, costing you probably something like �1400 for the batteries, and if you only actually get 6 hours of decent sun you'll need 2kW of solar panel costing �2k. If you have �3.5k+ to spend on developing your heating system for your greenhouse there would be better ways to spend it! Not to mention you need a lot of roof space for 2kW of PV panels and they can't be on your greenhouse or you block the sun!!

(You might be able to half those figures with bubble wrap, but will also need to increase the values if the GH is bigger than 4 x 6!)

I found online that 1g of candle wax contains 44-45kJ of energy. A tea light weighs about 15g. They take about 3 - 5 hours to burn so we'll assume 4 hours. That means they give off the same heat as a 46W heater (although some of that is 'lost' as light).

Provided you could switch the candles over every 4 hours without loosing heat opening the GH door! you could rig a tea light heater like in the video above using 6 lights and you'd just about be covering the heat requirement I initially assumed. At 1.75p each (Ikea) they'd cost you 32p a day. It'd take 8.5 years of continuous heating 12 hours a day to match the cost of the basic PV system I described which still didn't achieve what you needed! I'm assuming you only need to heat for 4 months a year so would take 25 years to break even and you need new batteries well before by then!

Compare that to a Parasene 4 heater (<�30) that will burn 4.5litres of paraffin over 7 days continuously. Costs about �6.00 if you shop around. That contains about 162MJ. Which is equivalent of about 267W. If you leave it running constantly for 4 months (16 weeks), you'd spend about �96 on my calculation plus the cost of the heater... That is more expensive than a tea light but more convenient as you don't have to change candles every 4 hours and would have no heat loss due to door opening every 4 hours!!

I see no reason why you shouldn't be able to get paraffin cheaper than �6/4.5litres. If you can get it straight from a pump it should be < 70p a litre so about half price.

[bazzaboy]

Apparently in Switzerland farms used to keep the cows on the ground level of the house and the heat warmed their little toesies.

Not just in Switzerland The Welsh Longhouses (and in Dartmoor and probably in the rest of the UK too) kept their cattle indoors through the winter, at one end of the longhouse - saved on the central heating bills

And in the room I'm writing this in north Notts (scroll on wall dated 14th year of George 3rd, 1774).... There used to be a ladder to go upstairs to bed (livestock hopefully not too good at climbing ladders) but a bit more modern now with a staircase in a (later added) cupboard..... A cottage I had in Suffolk still only had a ladder and I can still recall the fun and games when my elderly parents came to stay.... But more normally it would be the livestock below in harsh weather for a double whammy, protection for livestock, warmth for household...... Centrally heated now, duh, gorn soft...

Ace calculations polc1410, reading it kept me warm, the wonders of maths! You're absolutely right to draw attention to the difference between heating a greenhouse to maintain a standard temperature and finding ways of combating a damaging frost. Somewhere like Thanet Earth - see Thanet Earth (know the area well, grew up there) - controls every aspect of the weather (like the Welsh mushroom grower on last night's Countryfile) in order to grow stuff out of season, whereas I'm just trying to stop stuff getting killed off by water content freezing.... And just as you can get a frost pocket within a garden quite where you place something within the greenhouse space can make quite a difference too, as can a layer of bubble-wrap or fleece or even newspaper applied at night and removed in morning.

Getting late, time to climb the ladder....

[redser]

Was gonna try and work through the science of heating the air but it gets very complex trying to factor in the surface area of glass etc. But these guys have already done some of the work for you:

Calculate the power required to heat a Greenhouse by Hartley Botanic

For a 4 x 6 greenhouse I got roughly 282W heater being required to lift the temperature from 0C to 3C. That doesn't sound much in domestic terms.

However, if you are storing power from a solar panel you'd be thinking of a 12V battery. P=IV so 282=I x 12 so I = 23.5Amps. Thats a good amount of current! If you only needed to apply that heat for 12hours a day (you won't!) you'd need 282Ah of battery power. You should only run a 12V battery to about 40% of its stated capacity or you'll kill it very quickly. So you'd need 6 x 120Ah Leuisure Batteries ( c �100 each).

Assuming you had the other 12hours as perfect sun (you won't and you wont have the capacity to store it from Summer when you might have done)... you'd need a 300W solar panel. I found a 100W on amazon for �100 but looks like you normally pay more than that. But lets assume you can do it for�300 you are now at �1000 and still wont have a system that can actually do what you need it to because (a) It wouldn't be charged enough after 1 night of use before the next because you wont get 12hr of perfect sun (b) You may need more than 12hours of heat (c) You may need more than 3C increase in temp. If you get a realtively cold (rather than stupidly cold) winter say -5C for 8 hours and you maybe need to get the air temp to 2C to avoid frosting and then the outisde temp rises to -2C for the rest of the day (16hours). You probably need close to 700W during the really cold bit and 400W in the 'warmer' bit. That takes you to needing 1000Ah, costing you probably something like �1400 for the batteries, and if you only actually get 6 hours of decent sun you'll need 2kW of solar panel costing �2k. If you have �3.5k+ to spend on developing your heating system for your greenhouse there would be better ways to spend it! Not to mention you need a lot of roof space for 2kW of PV panels and they can't be on your greenhouse or you block the sun!!

(You might be able to half those figures with bubble wrap, but will also need to increase the values if the GH is bigger than 4 x 6!)

I found online that 1g of candle wax contains 44-45kJ of energy. A tea light weighs about 15g. They take about 3 - 5 hours to burn so we'll assume 4 hours. That means they give off the same heat as a 46W heater (although some of that is 'lost' as light).

Provided you could switch the candles over every 4 hours without loosing heat opening the GH door! you could rig a tea light heater like in the video above using 6 lights and you'd just about be covering the heat requirement I initially assumed. At 1.75p each (Ikea) they'd cost you 32p a day. It'd take 8.5 years of continuous heating 12 hours a day to match the cost of the basic PV system I described which still didn't achieve what you needed! I'm assuming you only need to heat for 4 months a year so would take 25 years to break even and you need new batteries well before by then!

Compare that to a Parasene 4 heater (<�30) that will burn 4.5litres of paraffin over 7 days continuously. Costs about �6.00 if you shop around. That contains about 162MJ. Which is equivalent of about 267W. If you leave it running constantly for 4 months (16 weeks), you'd spend about �96 on my calculation plus the cost of the heater... That is more expensive than a tea light but more convenient as you don't have to change candles every 4 hours and would have no heat loss due to door opening every 4 hours!!

I see no reason why you shouldn't be able to get paraffin cheaper than �6/4.5litres. If you can get it straight from a pump it should be < 70p a litre so about half price.

Great post. You can get 9 hour tea light in Ikea.
GLIMMA Unscented candle in metal cup - IKEA

I plan to use these. I also plan to section off a very small area at night with polythene (taking precautions of course) which should help. I'm also prepared to being young plants indoors on particularly cold nights. A combination of both should help things along I would imagine.

[polc1410]

4 times the price of a 4 hour job. Not sure what they'd have done to make it burn longer... does that mean it has a smaller flame (i.e. less heat) or is it a different (higher energy) wax?

Think the biggest issue will be on the very cold days when you want 24/7 heat and to change candle you need to open the door of the GH. Think you'll need to fabricate some sort of an 'airlock' like a porch just inside the door so that you open the GH door step inside. Close the door. Open an internal door and enter the GH proper... Reverse that to exit.

[redser]

Hi. They are just bigger candles. It is 4 times the price but it's still pennies. The area I am talking about would be much much smaller than a 4 x 6.
Personally I wouldnt even try to have heat 24/7. I would see that as a major luxury. I would only use it inside the sectioned off area or inside a blowaway inside the greenhouse. So opening the main door wont cause heat loss.
Personally Im not trying to overwinter tropicals or tender things. It would be to provide suplimental heat at night only for tender seedlings and young plants, probably through the second half of February onwards.